Bump height is very critical in semiconductor flip-chip packages. Under bump metal (UBM) size is directly related to bump height. However, for high density chip designs, the UBM size is limited by the pad size. The bump height will be limited by the UBM size in straight wall photoresist (PR) technology. A mushroom shaped bump height is one alternative method to improve the bump height without increasing the UBM size, however mushroom bump height control is not easy.
U.S. Pat. No. 6,211,052 to Farnworth describes a UBM process using photoresist, specifically Farnworth discloses a method for forming a UBM pad and solder bump connection for a flip-chip which eliminates at least two mask steps required in standard UBM pad forming processes when repatterning the bond pad locations.
U.S. Pat. No. 6,130,141 to Degani et al. describes techniques for applying UBM for solder bump interconnections on integrated circuit (IC) chips with aluminum (Al) bonding sites.
U.S. Pat. No. 6,015,652 to Ahlquist et al. describes a lift-off process for applying UBM for solder bump interconnections on interconnection substrates.
U.S. Pat. Nos. 5,903,058 and 5,736,456 both to Akram describe a method for forming UBM pads and solder bump connections for a flip-chip which eliminates at least one mask step required in standard UBM pad forming processes.
Accordingly, it is an object of an embodiment of the present invention to provide an improved method of fabricating bumps.
Other objects will appear hereinafter.
It has now been discovered that the above and other objects of the present invention may be accomplished in the following manner. Specifically, a UBM over a substrate having an exposed pad portion is provided. The UBM being in electrical contact with the pad portion. A first patterning layer is formed over the UBM. The first patterning layer being comprised of a photosensitive material sensitive to light having a first wavelength. A second patterning layer is formed over the first patterning layer. The second patterning layer being comprised of a photosensitive material sensitive to light having a second wavelength. The first patterning layer is selectively exposed with the light having the first wavelength, leaving a first unexposed portion substantially centered over the pad portion between first exposed portions. The second patterning layer is selectively exposed with the light having the second wavelength, leaving a second unexposed portion wider than, and substantially centered over, the first unexposed portion of the exposed first patterning layer. The second unexposed portion of the exposed second patterning layer being between exposed portions. The second unexposed portion of the exposed second patterning layer and the first unexposed portion of the exposed first patterning layer are removed to form opening. A solder plug is formed within the opening. The exposed portions of the exposed first patterning layer and the exposed portions of the exposed second patterning layer are removed. The solder plug is reflowed to form a solder bump.